Known reinforced-concrete building elements have reinforcements constituted as individual steel rods. The anchors are constituted as individual anchor elements in each of which a respective end of a respective rod is seated. The rods must be separated or fanned at the ends for connection to the anchors. When in this arrangement the rods, with or without protective coverings, are directly imbedded in the concrete of the building element, so it is not possible to pull them out and replace them at a later date. When the concrete/steel bond is not perfect, however, the steel is subject to corrosion. On the other hand when the reinforcement rods are received in a protective sleeve which is not filled with mortar, there is also a corrosion risk.
Thus in a standard prior-art system the reinforcement rods ar bonded along their full lengths to the surrounding concrete mass, whether or not the rods are surrounded with protective sleeves. This imbedding creates a good force-transmitting connection between the steel and the concrete and also protects the steel against corrosion. If the concrete/steel bond is bad in any locations the steel is subject to corrosion. In this arrangement the steel rods can be imbedded individually or as a bundle. Since all the rods are eventually fully imbedded in concrete, there is excellent lateral force transmission at the locations where the longitudinal tension in the rods is converted into transverse force. The prestressing can be done either to the rods as a group or individually.
It is also known to use multistrand cables as reinforcement elements (see "Betonwerk plus Fertigteiltechnik", 1984, pages 239 to 244). The cables each are formed by a plurality of rods or wires and run in a protective synthetic-resin sleeve which is filled around the cables with a grease or the like serving both as lubricant and anticorrosion agent. Such cables have not been used to date for the above-described type of construction elements.
Thus with this system the cables are set during manufacture into the concrete mass and are tensioned after the concrete has hardened. Each cable is seated at each end in a respective anchor element. The grease provides long-lived protection against corrosion and almost entirely eliminates friction between each cable and the respective surrounding sleeve. This system is called bond-free prestressing and is used almost exclusively for the prestressing of panels for high-rise structures, typically as floor plates. In such construction the necessary prestressing forces are small compared to those in bridge beams. Applications are also known for beams wherein the cables are distributed next to and above one another in a regular array. The cables run in a field at small spacings that are filled with concrete, but immediately before their ends the cables fan out in order to provide space for the end anchors.
It is also known from German patent document 3,734,954 (filed 15 October 1987 and assigned to Dyckerhoff & Widmann A.G.) to put several cables together as a group in a relatively large protective sleeve and to anchor this group in a common end piece, the cables being anchored together or individually. When the sleeve is straight it can be filled before or after the prestressing with mortar, but when it is curved and there will be lateral forces created by the prestressing so that the cables must be spaced and the interstices filled with mortar before the prestressing so that the lateral forces can be transmitted by the concrete to the surrounding structure. The bundle and its sleeve remain removable when they are not bonded into the mass by concrete. Such cable-type systems are nonetheless very difficult to fill completely with concrete or mortar.